This invention relates to a prescored foam board used in fabrication of composite panels, and more particularly to a method of manufacturing composite honeycomb core panels with a foam edge borders without manufacturing defects caused by outgasing from resin pre impregnated materials used for the face sheets of the panel.
A modification of airplane interior panels to improve the flammability resistance of the panels involves a change of the resin chemistry used in the resin preimpregnated materials used to form the face sheets of the panels. A phenolic resin system provides the desired reduction in flammability, but also creates manufacturing problems because of the increased volume of volatiles produced during the curing of the resin. This increased volume of volatiles must be accommodated in the manufacturing process to prevent generation of excessive pressure within the interior of the panel which prevents bonding of the face sheets of the panel to the panel core materials.
The panels are made up from a honeycomb core surrounded by a foam border, all bonded rigidly together between a face sheet, made of one or more plies of resin preimpregnated fibrous material (xe2x80x9cprepregxe2x80x9d), on each side of the panel. The elements that make up the panel are bonded together by the resin in the prepreg that forms the face sheets and the resin in the honeycomb core. The foam border is provided to give a clean uniform solid edge surface around the panel which the honeycomb material would not provide, allowing for sculpted or contoured edges.
Although the foam border performs its intended function as desired, it also tends to establish a barrier against egress of volatiles generated or otherwise present during the curing of the resin in the preimpregnated materials that form the face sheet and/or the honeycomb core during manufacturing. This is especially true when the panels are cured in an oven in which the heating of the panel to cure the resin in the preimpregnated face sheets tends to heat the panel first around its peripheral edges and on top, thereby flowing the resin at the peripheral edges first and establishing a seal between the face sheet and the foam border at a relatively early time in the cycle before all the volatiles in the other parts of the panel have been generated. Then, as the heating progresses and the interior and underside portions of the panel are heated in the oven, the resin cures, generating volatiles in the interior of the panel that can be trapped therein by the seal established earlier around the peripheral edge between the foam border and the face sheet.
One solution for the need to vent the interior of the panel during the generation of volatiles during curing of the resin is to score the foam border radially to provide gas channels leading from the interior of the panel to the outside edge of the foam border. This technique usually proved successful, but because of the variation in scoring depth and spacing between scoring marks, failures believed to be caused by excessive interior pressure within the panel during curing still occurred at an unacceptable rate. Also, a panel, appearing to have acceptable quality in the factory, would occasionally develop a delamination once at altitude because of the lower air pressure at altitude compared with the gas pressure within the sealed panel. Finally, the hand scoring technique was a very time consuming, costly, nonreproducable, and tedious task which the factory workers disliked. Moreover, when the hand scoring was done with a saw, it produced undesirable foam dust and created small cracks and tears in the foam which tended to act as stress risers, thereby lowering the strength that the foam edge border is expected to provide.
Accordingly, a need has existed for a prescored foam board material which can be used to make foam borders for composite panels, with an optimum scoring pattern which will provide gas flow channels from the interior of the panel to the exterior for venting of volatiles within the panel during fabrication. This pattern should be equally effective regardless of the angle at which the foam pieces are cut when the foam border is cut from the foam board stock. The scoring pattern should not weaken the foam board excessively. If possible the scoring of the gas channels in the foam surface should relax and at least partially fill in during the latter stages of manufacturing so that the panel interior can be sealed against intrusion of moisture or other contaminates during use in the airplane. Finally, the prescored foam should be no more expensive and preferably less expensive than the manual scoring method.
Accordingly, it is an object of this invention to provide an improved prescored foam board having a pattern of indentations that provides venting of volatiles generated within a composite panel during fabrication to prevent excessive gas pressure within the panel that could interfere with proper bonding of the panel elements during fabrication. Another object of the invention is to provide an improved method of making a composite panel using prescored foam borders to vent volatiles in the panel during curing of the resin used to make the panel. Still another object is to provide a method of making a prescored foam board to emboss an indentation pattern into the surface of the foam board used as a foam border on composite panels, which process preserves the strength of the board, avoids the generation of dust, and avoids creation of stress risers in the foam board which could affect its strength or service life. Yet another object of the invention is to provide a composite panel which is sealed around its periphery against intrusion of contaminates during use but which, during fabrication, had venting channels for escape of volatiles generated or otherwise present in the resin used in the materials used to make the composite panel. Still another object of the invention is to provide a method of prescoring foam board to produce gas channels for venting of volatiles generated within the core of the composite panel, which method is reliably effective in venting such volatiles and which also enables the channels to relax and facilitate sealing of the peripheral border of the panel against intrusion of contaminates.
These and other objects of the invention are attained in a prescored foam used in panel fabrication wherein the surface of the foam board has three series of parallel linear indentations, each of which lies at a angle 60xc2x0 offset from the other two series and which are rolled, stamped sawn or otherwise impressed into the surface of the foam by a laser, ultrasonic knife, patterned roller, flat die, sawing system, or the like. A roller embossing system is preferred because it which ramps the indentations into the board without creating stress risers. During fabrication, the gases generated within the core of the panel are vented through the indentations in the surface of the foam board out to the exterior of the panel, preventing the generation of excessive gas pressure within the panel. A vacuum is drawn inside the panel to draw the face sheets tight against the honeycomb core for reliable bonding. During heating, the foam material relaxes and springs back to partially fill in the indentations, and resin from the pre impregnated material which will form the face sheets flows to completely fill the partial indentations to reseal the peripheral edge of the panel. Any volatiles remaining or generated after the edge of the panel is sealed by the flowing of resin from the face sheet material are accommodated by the vacuum previously drawn in the interior of the panel to pull the face sheets against the honeycomb core material.